Diesel engines are power sources excellent in mileage and durability, and emit hydrocarbons (HC) and carbon monoxide (CO) in smaller amounts than gasoline engines. Thus, diesel engines are internal combustion engines suitable for mitigating a recent problem of global warming.
However, as compared with gasoline engines, diesel engines emit larger amounts of nitrogen oxides (NOx) and black smoke particulates to be generated upon combustion of air-fuel mixtures. This brings about a problem to be immediately solved by decreasing amounts of BOX and soot in exhaust gas.
In this respect, there has been known an exhaust gas purification system including: an oxidation catalyst disposed in a flow passage of exhaust gas from a diesel engine; and an exhaust gas purification filter disposed downstream the oxidation catalyst and having a ceramic porous wall acting as a filter element. In the exhaust gas purification system, NO, CO, HC, and the like in exhaust gas emitted from the diesel engine are oxidized by the oxidation catalyst, and soot contained in exhaust gas after oxidation is collected by pore portions of the porous wall of the exhaust gas purification filter as the exhaust gas after oxidation passes through the porous wall.
In the exhaust gas purification system, soot is collected by the pore portions within the wall and the surface of the wall, and the collected soot is removed by combustion with unburnt HC, CO, and the like. In this case, the filter may be damaged due to heat to be caused upon combustion removal of soot. This brings about a limitation of soot amount collectable by the filter, lest the filter is damaged by heat.
There has been thus disclosed the following technique for preventing damage due to heat (see S.A.E. article 2000-01-0473, “Passenger Car Serial Application of a Particulate Filter System on a Common Rail Direct Injection Diesel Engine”). This system is configured to estimate a soot amount accumulated within a filter by detecting a difference (pressure loss) between exhaust gas pressures at an upstream side and a downstream side of an exhaust gas purification filter, and an exhaust gas temperature thereat. Further, based on the thus estimated value, it is detected whether or not the exhaust gas purification filter is at a collection limit (which represents collection of soot reaching a regeneration limit). Upon detection of such a collection limit, the system thereafter supplies exhaust gas at a temperature higher than a soot inflammation temperature, to the exhaust gas purification filter through an oxidation catalyst, to thereby burn and remove the collected soot.
The above-mentioned exhaust gas purification filter regeneration method is configured to estimate a soot accumulation amount in the exhaust gas purification filter based on a pressure loss between the upstream side and downstream side of the filter, and to ascertain that the filter is at a soot accumulation limit. Namely, the exhaust gas purification filter regeneration method is configured to estimate a collection limit based on a relationship between the soot accumulation amount and the pressure loss in the exhaust gas purification filter.
In the above, regeneration of the exhaust gas purification filter is to be achieved by exhaust gas from the diesel engine, thereby causing a possibility that soot is not fully removed by combustion thereof at one time and is thus left as residual soot in the filter, depending on a driving condition of the engine.
The residual soot tends to affect an ascertaining operation of a soot accumulation limit upon conducting the next regeneration, thereby making it necessary to ascertain the residual soot accumulation amount.
However, in the exhaust gas purification filter regeneration method, once regenerating the catalyst causes partial combustion, movement of soot, and the like, thereby making it impossible to accurately recapture a pressure loss relative to a soot accumulation amount. This makes it difficult for the exhaust gas purification filter regeneration method to estimate an accurate residual soot accumulation amount from a pressure loss.